1. Field of the Invention
This invention relates to a process for the agglomeration of alumina containing material substantially comprising Al.sub.2 O.sub.3.nH.sub.2 O where n is in the range of from zero to 3. The invention also relates to agglomerated granules produced by that process.
2.Description of the Prior Art
Fine powder or dust of such alumina containing material (herein called "alumina powder") is difficult to handle and has poor flow characteristics. As a consequence, by-product alumina powder from the Bayer process presents difficulties. In the Bayer process, precipitated alumina trihydrate is filtered, dried and calcined to yield high purity alumina product of a relatively narrow size spectrum, for use in an electrolytic smelting operation. By-product alumina powder, referred to as fines, superfines and sometimes as ESP dust, is recovered by multicyclones and/or electrostatic precipitator collectors from the calcining stage and typically has an average particle size of less than 30 .mu.m. In addition to being difficult to handle and having poor flow characteristics, by-product alumina powder can not be readily digested if recycled to the hot caustic digestion stage of the Bayer process. Also, if added to the alumina product for use in the smelting operation, it increases the size range and dustiness of the product.
There is a need to be able to agglomerate alumina powder into a coarser product of a size range which approximates the preferred size range for the smelting operation. However, there also can be benefit in ceramics manufacturing in being able to agglomerate fine alumina powder, whether this is ESP dust or is from another source.
In ceramics manufacturing, fine micron sized ceramic powders are agglomerated by spray drying, using an organic polymer, such as PVA, as a binder. The purpose, however, is to make weak granules that act as a flowable precursor to facilitate pressing of low porosity green bodies of ceramics, prior to firing. However, these granules are essentially friable and weakly bonded, and may be degraded when handled or transported. Thus, agglomeration with such organic polymers is of limited benefit.
We have found that alumina powder can be agglomerated by use of a suitable inorganic binder.